Merge LKMM and RCU commits

<li>	<a href="#Hotplug CPU">Hotplug CPU</a>.

<li>	<a href="#Scheduler and RCU">Scheduler and RCU</a>.

<li>	<a href="#Tracing and RCU">Tracing and RCU</a>.

<li>	<a href="#Accesses to User Memory and RCU">

Accesses to User Memory and RCU</a>.

<li>	<a href="#Energy Efficiency">Energy Efficiency</a>.

<li>	<a href="#Scheduling-Clock Interrupts and RCU">

	Scheduling-Clock Interrupts and RCU</a>.

<p>

It is possible to use tracing on RCU code, but tracing itself

uses RCU.

For this reason, <tt>rcu_dereference_raw_notrace()</tt>

For this reason, <tt>rcu_dereference_raw_check()</tt>

is provided for use by tracing, which avoids the destructive

recursion that could otherwise ensue.

This API is also used by virtualization in some architectures,

The tracing folks both located the requirement and provided the

needed fix, so this surprise requirement was relatively painless.

<h3><a name="Accesses to User Memory and RCU">

Accesses to User Memory and RCU</a></h3>

<p>

The kernel needs to access user-space memory, for example, to access

data referenced by system-call parameters.

The <tt>get_user()</tt> macro does this job.

<p>

However, user-space memory might well be paged out, which means

that <tt>get_user()</tt> might well page-fault and thus block while

waiting for the resulting I/O to complete.

It would be a very bad thing for the compiler to reorder

a <tt>get_user()</tt> invocation into an RCU read-side critical

section.

For example, suppose that the source code looked like this:

<blockquote>

<pre>

 1 rcu_read_lock();

 2 p = rcu_dereference(gp);

 3 v = p->value;

 4 rcu_read_unlock();

 5 get_user(user_v, user_p);

 6 do_something_with(v, user_v);

</pre>

</blockquote>

<p>

The compiler must not be permitted to transform this source code into

the following:

<blockquote>

<pre>

 1 rcu_read_lock();

 2 p = rcu_dereference(gp);

 3 get_user(user_v, user_p); // BUG: POSSIBLE PAGE FAULT!!!

 4 v = p->value;

 5 rcu_read_unlock();

 6 do_something_with(v, user_v);

</pre>

</blockquote>

<p>

If the compiler did make this transformation in a

<tt>CONFIG_PREEMPT=n</tt> kernel build, and if <tt>get_user()</tt> did

page fault, the result would be a quiescent state in the middle

of an RCU read-side critical section.

This misplaced quiescent state could result in line 4 being

a use-after-free access, which could be bad for your kernel's

actuarial statistics.

Similar examples can be constructed with the call to <tt>get_user()</tt>

preceding the <tt>rcu_read_lock()</tt>.

<p>

Unfortunately, <tt>get_user()</tt> doesn't have any particular

ordering properties, and in some architectures the underlying <tt>asm</tt>

isn't even marked <tt>volatile</tt>.

And even if it was marked <tt>volatile</tt>, the above access to

<tt>p-&gt;value</tt> is not volatile, so the compiler would not have any

reason to keep those two accesses in order.

<p>

Therefore, the Linux-kernel definitions of <tt>rcu_read_lock()</tt>

and <tt>rcu_read_unlock()</tt> must act as compiler barriers,

at least for outermost instances of <tt>rcu_read_lock()</tt> and

<tt>rcu_read_unlock()</tt> within a nested set of RCU read-side critical

sections.

<h3><a name="Energy Efficiency">Energy Efficiency</a></h3>

<p>

	CONFIG_PREEMPT_RCU case, you might see stall-warning

	messages.

	You can use the rcutree.kthread_prio kernel boot parameter to

	increase the scheduling priority of RCU's kthreads, which can

	help avoid this problem.  However, please note that doing this

	can increase your system's context-switch rate and thus degrade

	performance.

o	A periodic interrupt whose handler takes longer than the time

	interval between successive pairs of interrupts.  This can

	prevent RCU's kthreads and softirq handlers from running.

			RCU_BOOST is not set, valid values are 0-99 and

			the default is zero (non-realtime operation).

	rcutree.rcu_nocb_leader_stride= [KNL]

			Set the number of NOCB kthread groups, which

			defaults to the square root of the number of

			CPUs.  Larger numbers reduces the wakeup overhead

			on the per-CPU grace-period kthreads, but increases

			that same overhead on each group's leader.

	rcutree.rcu_nocb_gp_stride= [KNL]

			Set the number of NOCB callback kthreads in

			each group, which defaults to the square root

			of the number of CPUs.	Larger numbers reduce

			the wakeup overhead on the global grace-period

			kthread, but increases that same overhead on

			each group's NOCB grace-period kthread.

	rcutree.qhimark= [KNL]

			Set threshold of queued RCU callbacks beyond which

	rcutorture.verbose= [KNL]

			Enable additional printk() statements.

	rcupdate.rcu_cpu_stall_ftrace_dump= [KNL]

			Dump ftrace buffer after reporting RCU CPU

			stall warning.

	rcupdate.rcu_cpu_stall_suppress= [KNL]

			Suppress RCU CPU stall warning messages.

LINUX KERNEL MEMORY CONSISTENCY MODEL (LKMM)

M:	Alan Stern <stern@rowland.harvard.edu>

M:	Andrea Parri <andrea.parri@amarulasolutions.com>

M:	Andrea Parri <parri.andrea@gmail.com>

M:	Will Deacon <will@kernel.org>

M:	Peter Zijlstra <peterz@infradead.org>

M:	Boqun Feng <boqun.feng@gmail.com>

	return 0;

}

static DECLARE_COMPLETION(cpu_died);

/*

 * called on the thread which is asking for a CPU to be shutdown -

 * waits until shutdown has completed, or it is timed out.

 */

void __cpu_die(unsigned int cpu)

{

	if (!wait_for_completion_timeout(&cpu_died, msecs_to_jiffies(5000))) {

	if (!cpu_wait_death(cpu, 5)) {

		pr_err("CPU%u: cpu didn't die\n", cpu);

		return;

	}

	 * this returns, power and/or clocks can be removed at any point

	 * from this CPU and its cache by platform_cpu_kill().

	 */

	complete(&cpu_died);

	(void)cpu_report_death();

	/*

	 * Ensure that the cache lines associated with that completion are